59^ On the Lifiti'wn, R'Jiix'ion, and Colours of Light- 



decompounded fcattered light, and dilutes it. 2d, That the nearer to the perpendicular tlie 

 rays arc incident, the more light will be reflected to the focus, and confequcntly the lefs 

 •w-ill dilute and weaken the rings. 3d, That the thinner the mirror is, or the nearer tlie two 

 fiufaces are, the bro;ider will the rings be. 4th, Th.it thfi rings farther from the focus will be 

 broader. And lallly, that wlicn homogeneous light is reflcfled the fringes or images will 

 be larger, and f;iriher from one another in red than in any other primary colour. All 

 which is perfciftly <:onfi(U:nt with the experiments of Newton and Chaulnes. There is 

 only one difficulty that may be darted to this explanation : How happens it that the colours 

 (nrade by tlie mirror) arc always circular .■' We anfwer. It is owing to the manner of po- 

 lilhing the concave mirror, which is laid between a convex and concave plate, and then 

 turned round (with putty, or melted pilch) in the very dire(5\ion in which the rings are. 

 If it fl.ould be alked. Why does the thicknefs of the mirror influence the breadth of the rings 

 exactly in the inverfe fubduplicate ratio ? we anfwer, That to a certain didance from the 

 point of incidence (and the rays are never fcattered far from it) this is demonltrable to 

 liold as a property of mathematical lines in general. 



H.iving found that the fringes by flexion are images, of the luminous body*, I thought 

 that from this confidcration a method of determining the different degrees of flexibility of 

 the difterent rays might be deduced, flrailar to that which I had formerly ufed for deter- 

 mining their degrees of reflexibility -J-. I therefore made the following experiment : 



ObJ. I i-X Having let into my darkened chamber a (Irong beam of the fun's light, through 

 a hole j^„th of an inch in diameter, I held a hair at four feet from the hole ; and receiving 

 the fliadow at two feet from the hair, I drew a line acrofs the tniddle of the coloured 

 images, and pointed ofl" in each the divifions of the colours, as nearly as I could obfcrve ; 

 and repeating tliC obfervation feveral times, and at different ilillances, 1 found, by the fame 

 way I had formerly done in my experiment on reflexibility, tliat the axis, or line drawn 

 through the middle of each, was divided inverfely according to the intervals of the chords 

 which found the notes in an o£lave, iit, re, mi, pi, la,fa,fi, iit. Dut as tiie meafures in 

 thcfe experiments were very minute, and the operations of confequence liable to inaccu- 

 racy, I thought proper to try the thing by anotJier teil. 



(Jhj. 13. The lun {hining into the room, as before, I placed at tlie hole an hollow prifm 

 made of fine plate glafs and filled with pure water, its refracking angle being 5 5 degrees. 1 he 

 fpeclruni was thrown on an horizontal chart eight feet from the window ; and at four feet 

 from the prifm there was placed in the rays a rough black pin j'tth of an inch in diameter. 

 'Hie fliadow in the fpeftrum was bounded by hyperbolic fides, as before defcribed ; and 

 drawing a line, which might be the axis of the fliadow, and pafs prccifely through its 

 middle, I marked on one fule fix or eight points of the fliadow's outline in each fet of rays 5 

 and this being often repeated at different diftanccs and in different ftiadows, the pofition of 

 the'a>('is remaining the fame, the curves formed by jrining the points were all parallel ; 

 which fliows that each fine of inflexion taken apart has a given ratio to the fine of incidence. 

 I afterwards divided the a'\is according to the mufical intervals, and thus found where each 

 colour of the fpeclrum had terminated, in what colour each part of the ftiadows had been, 



• p .,,■ ■_■ \ ■'■ Pipr- J'- . frnumhertdin tl'.e orij; nal. N. 



and 



